Flip-chip technology has recently moved from a specialized packaging application to mainstream production as an important method for improving circuit densities, reliability and cost in miniaturized electronic products. The use of an underfill encapsulant beneath the flip-chip die is generally necessary to improve reliability by reducing the strain on the solder bumps during thermal cycling imposed by coefficient of thermal expansion (CTE) differences between the IC die and the substrate. By adhering to all surfaces under the die, the underfill adhesive makes the die-adhesive-substrate system significantly stiffer. The underfill adhesive also keeps the solder bump in hydrostatic compression, thus increasing fatigue endurance and holding the bump intact under strain.
Both capillary underfill and no-flow underfill processes are known. Basically, conventional capillary underfill dispensing takes place in the production line after the reflow process, whereas no-flow underfill is dispensed before placement of the IC die and is designed to cure during the reflow process. The capillary underfill process requires an additional cure oven and a means to flux the area before reflow. Both processes require a dispenser.
A conventional wafer scale bonding method for attaching an IC die to a substrate (e.g., PCB) based on capillary underfill comprises soldering the IC die (e.g., as a flip chip) to the substrate and then injecting an underfill material between the IC die and the substrate. Capillary flow causes the underfill to seal the area between the IC die and the substrate that is not occupied by the soldered areas of connection. An underfill cure step then occurs. Injection of underfill is known to result in a significant density of underfill voids, particularly when the geometries (e.g., of the gaps) to be filled are relatively small. Voids typically result in a loss of reliability. Moreover, a drawback with soldering the IC die to the substrate is that the various components contract at different rates during bonding.
Another method of attaching an IC die to a substrate is a “no-flow” underfill method which generally comprises thermal compression bonding (TCB) IC die to the substrate. A typical TCB process includes covering the solder bumps on a substrate with an underfill precursor material and then positioning solder bumps on an IC die in a flip chip configuration against the solder bumps on the substrate. A pressing step simultaneously applies heat to the solder bumps, and causes solder interconnect reflow and underfill cure. One of the advantages of TCB over conventional capillary flow processes is that the extra processing steps that are associated with a capillary flow process (e.g., flux application, flux residue cleaning and secondary thermal curing of the underfill) are eliminated. However, TCB like conventional wafer scale bonding, is known to result in generation of a significant density of reliability reducing underfill voids, many of which can be of significant size.
Conventional microelectronics assembly equipment for pressing to form metallic joints generally comprises a clam-shell arrangement, where the first and second clam shell portions of the “clam” are coupled by a hinge. The clamming motion is used to simultaneously press together a plurality of IC die that are on the surface of a wafer or package substrate by applying a compressive force that is oriented vertical with the respect to the surface of the wafer or package substrate. Heat applied during the pressing step provides heat for the solder interconnect reflow and underfill cure to occur simultaneously.